clarify caveats in README

[timotheecour]

Pre-existing docs were misleading in several ways (see below), this PR clarifies the README and also shows caveats with using monorepo.load_package (in particular that it breaks modularity, can cause module clashes and duplicate modules that can result in silent runtime bugs, see PR README).

IMO monorepo.load_package should be deprecated and this PR suggests simple and standard alternatives to achieve same goals (using uniform import syntax from anywhere in monorepo) by simply adding monorepo root to PYTHONPATH (either programmatically or on command line).

• foo.bar_function() in the README gave the impression it would work for modules but it only works for packages (eg if bar_function() is in foo/init.py)
• foo = monorepo.load_package("example.package.foo") was mis-leading as the returned name is not used, eg:

monorepo.load_package("example.package.foo")
from foo import mymodule1 # ok
foo = monorepo.load_package("example.package.foo")
from foo import mymodule1 # ok
foo2 = monorepo.load_package("example.package.foo")
from foo2 import mymodule1 # error: foo2 not defined

There is code in the wild that falls into this trap, eg starfo*/read_data_test.py: char_dataset = monorepo.load_package("common.dataset.dataset")